Block Mapping on Reserved Field

[pickypg]

When trying to change the behavior of certain reserved fields, a user may be tempted to try to change their mapping:

PUT my_index
{
  "mappings": {
    "_doc": {
      "_field_names": {
        "type": "keyword"
      }
    }
  }
}

This doesn't work (it remains mapped as text in this case, which coincidentally seems wrong), but the request is accepted. When fetching the mapping, this attempted alteration does not exist in the response.

Relates to #8143.

[elasticmachine]

Pinging @elastic/es-search-aggs

[pickypg]

As a side note, if you try to put it into the properties section of the type, then it does properly reject the field with Field [_field_names] is defined twice in [_doc].

It seems like we need to become a little stricter about unknown fields outside of the properties block.

[jtibshirani]

Thanks @pickypg for reporting this and sorry for the very late response! I dug a bit into this behavior and had a few clarifications:

• The correct way to configure _field_names is indeed outside the properties block (https://www.elastic.co/guide/en/elasticsearch/reference/7.1/mapping-field-names-field.html). We correctly reject invalid fields outside the properties block, returning the error ""Root mapping definition has unsupported parameters: ..."
• As you noted, it is not possible to update metadata field mappings, as we throw an error of the form "Field ... is defined twice in ...".

I think a big reason your original example caused confusion is that we accept "type": "keyword" in the _field_names configuration, even though this is not a valid argument. This bug seems to be specific to the 'type' key, since we correctly reject other invalid keys. I'll open a PR to fix this leniency.